Since a solid polymer fuel cell is easy to miniaturize and lighten, the research & development has been earnestly advanced as power sources for various electronic apparatuses such as portable apparatuses.
The solid polymer fuel cell contains a membrane and electrode assembly (MEA) in which a solid polymer electrolyte membrane is put between an anode and a cathode. A fuel cell of a type that fuel is directly supplied to the anode is referred to as a direct type fuel cell, and the supplied fuel is dissociated on catalyst that is held on the anode. Consequently, protons, electrons and intermediate products are generated. Moreover, in the fuel cell of this type, the generated protons pass through the solid polymer electrolyte membrane to a cathode side, and the generated electrons move through an external load to the cathode side. Then, they react with oxygen in air on the cathode. Consequently, the electric power is generated. For example, in a direct methanol type fuel cell (hereinafter, to be referred to as DMFC) in which a methanol aqueous solution is used in its original state as the fuel, the anode reaction represented by the following formula is carried out on the anode:CH3OH+H2O→CO2+6H4+6e−Then, the cathode reaction represented by the following formula is carried out on the cathode:6H++6e−+ 3/2O2→3H2OThat is, in the DMFC, theoretically, the methanol of 1 mole and the water of 1 mole react to generate the reaction product (carbon dioxide) of 1 mole. At this time, since hydrogen ions and electrons are also generated, the theoretical concentration of the methanol in the methanol aqueous solution serving as the fuel becomes about 70 vol % in a volume %.
However, when the methanol concentration supplied to the anode becomes high, it is known that a “crossover” phenomenon is caused in which the methanol passes through the solid polymer electrolyte membrane without any contribution to the anode reaction so that the electric generation capacitance and the generated electric power are reduced. When the crossover becomes severer, the following troubles are like to cause that: (1) the output (voltage) is decreased, (2) the use efficiency of the fuel becomes low, (3) since the calorific value is increased, the temperature of the MEA is increased, which increases the fuel temperature beyond necessity, and further increases the crossover, and consequently involves the further temperature increase.
In order to make the output of the MEA high, the proton conduction of the electrolyte membrane is required to be made high. However, this also leads to the fact that the transmission velocity of the methanol becomes high. Thus, when the necessary output is tried to be reserved, the influence of the crossover is actually received, though the methanol aqueous solution of about 20 vol % is used. On the contrary, the use of the methanol aqueous solution of the lower concentration makes the reduction of the crossover easier. However, when the methanol aqueous solution of the low concentration is used as the fuel, the electric generation amount per unit mass of the fuel is decreased, which results in a problem that the energy density of the solid polymer fuel cell cannot be increased. Thus, in order to obtain the solid polymer fuel cell whose energy density is high, it is desired to use the fuel that is as close as possible to the theoretical optimal methanol concentration (70 vol %) while the crossover is suppressed.
As the DMFC technique for suppressing the crossover, a fuel cell is known which contains a gas liquid separation membrane as a fuel vaporization layer in front of the anode portion of the MEA to vaporize the supplied fuel.
According to the description of Japanese Patent Application Publication (JP-P2000-106201A), an effect is described in which, since the fuel is vaporized and supplied as mentioned above, the gas fuel inside the fuel vaporization layer is held in a substantially saturated state. Thus, for a consumption amount of the gas fuel in the fuel vaporization layer caused by the battery reaction, the liquid fuel is vaporized from the fuel permeation layer. Moreover, based on the vaporization amount, the liquid fuel is introduced into a cell by a capillary force. In this way, since a fuel supply amount is related to the fuel consumption amount, there is almost no fuel that is exhausted to outside the battery without any reaction. Then, differently from the conventional liquid fuel cell, the process group on the fuel outlet side is not required.
In short, as shown in FIG. 4, a fuel permeation layer 106 for introducing the fuel into the battery through the capillary force and a fuel vaporization layer 107 that is arranged between an anode 102 and the fuel permeation layer 106 and vaporizes the fuel introduced into the cell and then supplies the gaseous fuel to the anode. A plurality of fuel permeation layers 106, fuel vaporization layers 107 and electric generators 104 are laminated through separators 105. Thus, a stack 109 serving as a battery body is configured. The fuel inserted into a liquid fuel introduction path 110 is supplied from the side of the stack 109 to the fuel permeation layer 106 through the capillary force, and further vaporized in the fuel vaporization layer 107 and then supplied to the anode 102. The separator 105, the fuel permeation layer 106 and the fuel vaporization layer 107 carry out the function as an electric collector for transferring the generated electrons. Therefore, for example, the fuel permeation layer 106 is made of a carbon conductive material.
The foregoing fuel cell may be configured such that the mixture solution whose mole ratio between the methanol and the water is 1:1 is used as the fuel, and the supply of the fuel to the liquid fuel introduction path 110 from a fuel tank is carried out by the natural falling resulting from the installation in which the tank is placed above the electric generator or the extrusion of the fuel with the internal pressure inside the tank or the like or may be configured to pull out the fuel through the capillary force of the liquid fuel introduction path 110.
Japanese Patent Application Publication (JP-P2001-15130A) describes a mechanism in which a porous body whose surface made of a material having a thermal conductivity of 20 W/m·K or more is made of fluorine resin is used for a separation membrane, and the heat generation of the MEA is used, and the liquid fuel is vaporously supplied by the vaporization heat.
When the configurations disclosed in Japanese Patent Application publications (JP-P2000-106201A, and JP-P2001-15130A) were further considered, the inventors of this application discovered that there were the following problems and the stable electric generation could not be carried out under its condition.
At first, the configuration in Japanese Patent Application Publication (JP-P2000-106201A) is assumed such that a mixture solution whose mole ratio of the methanol and the water is 1:1 is used, and an internal pressure inside a tank allows the liquid fuel to be supplied to the fuel vaporization layer 107. However, the inventors of this application discovered that the stable fuel supply could not be carried out under the configuration. In short, in the fuel supply through such capillary force, when the methanol aqueous solution of a high concentration is used, the methanol aqueous solution whose concentration is higher than the liquid fuel is supplied due to a balanced condition between the liquid phase and the gas phase. That is, it is difficult to carry out the stable electric generation in which the methanol aqueous solution of the high concentration is used. Also, in this fuel supplying method, it is difficult to carry out the perfect vaporization supply, and a portion supplied as the liquid causes the crossover. With the above reasons, it is difficult to use the methanol aqueous solution of the high concentration as the fuel.
In the fuel cell of Japanese Patent Application Publication (JP-P2001-15130A), a porous membrane is used on which a water repelling process has been performed. Thus, as compared with Japanese Patent Application Publication (JP-P2000-106201A), there is almost no case that the liquid fuel is directly swept out. Also, since the fluorine-based polymer such as poly tetra fluoro ethylene (PTFE) can be used which is chemically stable, it is superior in long-term reliability. However, as well known, a hydrophobic porous body is known as a gas liquid separation membrane material for concentrating and separating an alcohol aqueous solution. That is, also, in this case, when the methanol of the high concentration is used, the permeation of the methanol is major. Therefore, there is a problem that because of a lack of the water and the methanol crossover increase to the cathode electrode, the sufficient voltage could not be obtained. Actually, the actual use range is below 20 vol %.
Also, as the related art, Japanese Patent Application Publication (JP-P2004-79506A) describes a technique for providing a liquid fuel cell that is small in size and can stably attain an electric generation. Also, Japanese Patent Application Publication (JP-P2002-289224A) describes a technique whose subject is to provide a fuel cell that can solve a problem of deterioration in an electric generation efficiency caused due to a gas generation near an output terminal and attain a high output. Also, Japanese Patent Application Publication (JP-P2000-268836A) describes a technique for providing an electric generation apparatus that can prevent the crossover of the liquid fuel and can stably supply the fuel to a negative electrode though the liquid fuel is decreased or the upper lower positional relation is varied.